Method of coating copper conductors on polyimide with a corrosion resistant metal, and module produced thereby

ABSTRACT

A module, for interconnecting integrated circuits, is comprised of a substrate, a layer of polyimide disposed thereon, and a plurality of spaced apart copper conductors on the layer of polyimide. This module is dipped into a solution of palladium chloride, thereafter it is dipped into a solution of sodium hydroxide, and thereafter it is dipped into an electroless plating solution for cobalt. As a result, a film of cobalt is deposited on all exposed surfaces of the copper conductors, but no cobalt is deposited on the exposed polyimide surface between the conductors. This cobalt film protects the copper conductors from corrosion, and it does not short the conductors together.

BACKGROUND OF THE INVENTION

This invention relates to methods for fabricating modules whichinterconnect integrated circuits with patterned layers of copperconductors that are separated by layers of polyimide; and moreparticularly, it relates to methods of coating the copper conductorssuch that they do not corrode.

In the prior art, one module of the type to which this invention relatesis described in an article entitled "Copper/Polyimide Systems for HighPerformance Packaging", IEEE Transactions on Components, Hybrids &Manufacturing Technology, Volume 7, No. 4, December, 1984, by H. Vora,et al. As described in this article, the conductors are formed on anunderlying layer of polyimide by sputtering thereon, (1) an unpatternedlayer of chromium, (2) an unpatterned layer of copper on the chromium,and (3) an unpatterned layer of chromium on the copper. This Cr/Cu/Crstructure is then patterned in a conventional fashion by means ofphotoresist and a wet etch. Then the resulting patterned Cr/Cu/Crconductors are covered with an overlying layer of polyimide. All of theprocess steps are then repeated to form additional Cr/Cu/Cr conductorlayers which are separated by polyimide layers.

However, a problem with the above described interconnect structure isthat no chromium covers the sidewalls of the patterned Cr/Cu/Crconductors. Consequently, the copper in the sidewalls corrodes when itis covered by the overlying polyimide layer. This corrosion occursbecause water is given off when the overlying polyimide layer is cured,and because the curing process occurs at a high temperature (e.g., 300°C.).

To avoid this corrosion problem, silicon dioxide rather than polyimidecould be used to cover the copper conductors. But silicon dioxide cannotbe deposited over the conductors as smoothly as polyimide. Polyimide isspun on in a liquid form and then cured, and this spinning motionsmooths out the polyimide surface. By comparison, silicon dioxide isdeposited by a chemical vapor. An insulating layer with a smooth surfaceis very useful since it allows alternate layers of copper conductors andinsulator to be stacked on top of each other many times.

It has been considered by the present inventor that it would be verydesirable to be able to pattern a layer of copper conductors on theunderlying polyimide layer, and then electrolessly plate just theexposed copper surfaces with a material that resists corrosion. However,prior art electroless plating methods plate the polyimide along with thecopper. Consequently, the copper conductors become shorted together andthereby make the interconnect structure useless.

Accordingly, a primary object of the invention is to provide a method offabricating a module which electrically interconnects integratedcircuits with copper conductors on polyimide layers in which all of theabove problems are resolved.

BRIEF SUMMARY OF THE INVENTION

In one preferred process which the invention covers, the above object isachieved by providing a substrate, a layer of polyimide disposedthereon, and a plurality of spaced apart copper conductors on the layerof polyimide. This structure is dipped into a solution of palladiumchloride, thereafter it is dipped into a solution of sodium hydroxide,and thereafter it is dipped into an electroless plating solution forcobalt. As a result, a film of cobalt is deposited on all exposedsurfaces of the copper conductors, but no cobalt is deposited on theexposed polyimide surface between the conductors. This cobalt filmprotects the copper conductors from corrosion, and it does not short theconductors together.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the invention are described in theDetailed Description in conjunction with the accompanying drawingswherein:

FIGS. 1, 2, 3, 4, 5, and 6 illustrate various stages of a fabricationprocess which occurs in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the FIGS. 1 thru 6, a preferred process for fabricatingthe same will be described in detail. First, beginning with FIG. 1, itshows the structure with which the fabrication process starts. Thisstructure includes a susbstrate 10 which has a substantially flat majorsurface 10a, a polyimide layer 11 which covers surface 10a, andpatterned copper conductors 12 which are disposed on the polyimidelayer.

Substrate 10 may be made of any semiconductor material such as siliconor germanium; or alternatively, it may be made of nonsemiconductormaterials, such as ceramic or sapphire. Further, substrate 10 may haveany desired thickness, and surface 10a may have any desired shape. Forexample, surface 10a may be square and one-quarter inch on a side, or itmay be round and five inches in diameter.

Insulating layer 11 is comprised of polyimide and may have any desiredthickness. Also, the copper conductors 12 can have any desiredthickness, and they can be patterned to any desired shape. Suitably, theconductors 12 are shaped in a conventional fashion by disposing anunpatterned layer of copper on polyimide layer 11, photographicallypatterning a layer of photoresist on the copper layer, and removing thecopper which is not covered by the photoresist with a chemical etchant.

After the FIG. 1 structure is fabricated, the entire structure iscompletely dipped into a solution which contains a catalyst forelectrolessly plating a corrosion resistant metal. This is shown in FIG.2. One preferred solution is 0.05 grams of palladium chloride per literof water and 0.1 milliliter of concentrated hydrochloric acid per literof water. Here, palladium is a catalyst for electrolessly plating cobaltor nickel. Using this solution, the dipping preferably lasts five tofifteen seconds and occurs at room temperature.

Due to this dipping step, the copper surfaces are "activated" whichmeans they will readily plate if they are immersed in an electrolessplating solution. However, the inventor's experiments show that thepolyimide surfaces 11a will also plate along with the copper surfaces atthis stage of the process. Whether this is due to the above dipping stepor due to the previous etching of the copper 12 or an inherentcharacteristic of the polyimide is unknown. In any case, the "dots" 13in FIG. 2 indicate the "active" surface.

Next, as shown in FIG. 3, the above processed structure is dipped into asolution which contains a chemical agent that selectively "deactivates"the polyimide but not the copper. By "deactivate" is herein meantsubstantially delay or reduce the tendency to plate. One preferredsolution is two to ten grams of sodium hydroxide per liter of water.Here, the hydroxide is the agent which performs the selectivedeactivation. Using this solution, the immersion preferably occurs atroom temperature and lasts fifteen to sixty seconds. Selectivedeactivation is indicated in FIG. 3 by the "dots" 13' which are on onlycopper surfaces.

Following the step of FIG. 3, the resulting selectively deactivatedstructure is immersed into a solution for electrolessly plating acorrosion resistant metal. In one such solution for plating cobalt, theprimary active ingredients are cobaltous sulfate and a reducing agent,dimethylamine borane. Preferably, the immersion in the plating solutionlasts for about five to sixty seconds at 40° C.-65° C. As a result, thecorrosion resistant metal 14 is deposited over all of the exposedsurfaces of the copper conductors 12, but nothing is deposited on theexposed surface of the polyimide layer 11. This is shown in FIG. 4.

Thereafter the FIG. 4 structure is covered with a layer of polyimide 15.This is done in a conventional fashion by placing liquid polyimide onthe FIG. 4 structure, spinning the liquid polyimide to form asubstantially flat polyimide layer, and curing the polyimide at atemperature of about 300° C. During this step, no corrosion of thecopper conductors 12 occurs because all of the copper surfaces which lienext to the newly cured polyimide layer are covered with the protectantfilm 14.

All of the above described steps can then be repeated on the FIG. 5structure to form an embodiment which contains multiple layers of copperconductors, each of which is coated with a protectant film. One suchembodiment is shown in FIG. 6. There, reference numeral 22 indicates anupper layer copper conductor; reference numeral 23 indicates the activesurface; reference numeral 24 indicates a corrosion resistant film; andreference numeral 25 indicates a polyimide layer over the film 24.

A preferred process according to the invention, as well as the resultinginterconnect module, has now been described in detail. In addition,however, many changes and modifications can be made to these detailswithout departing from the nature and spirit of the invention.

For example, the solution concentrations and immersion times are notlimited to the preferred ones which are given above. Also, various otherelectroless plating catalysts may be substituted for palladium. Also, inthe selective deactivation step of FIG. 3, other agents, such aspotassium hydroxide, can be used. Further, in the plating step of FIG.4, other metals, such as nickel or gold-covered nickel, can beelectrolessly plated. Also, in the steps of FIGS. 2, 3 and 4, the copperconductors need not be pure copper but can include a thin underlyinglayer of chromium or other suitable material which helps the copperadhere to the polyimide.

Accordingly, it is to be understood that the invention is not limited tothe preferred above details but is defined by the appended claims.

What is claimed is:
 1. A method of coating spaced apart copperconductors which lie on one layer of polyimide with a corrosionresistant metal without shorting said conductors together; said methodincluding the steps of:dipping said one layer of polyimide and saidcopper conductors which lie thereon into a solution of a catalyst forelectrolessly plating said metal; immersing, subsequent to said dippingstep, said one layer of polyimide and said conductors which lie thereoninto an hydroxide solution; bathing, subsequent to said immersing step,said one layer of polyimide and said conductors which lie thereon in anelectroless plating solution for said metal.
 2. A method according toclaim 1 wherein said solution in said dipping step contains palladium assaid catalyst.
 3. A method according to claim 1 wherein said hydroxidein said immersing step is sodium-hydroxide.
 4. A method according toclaim 1 wherein said hydroxide in said immersing step ispotassium-hydroxide.
 5. A method according to claim 1 wherein saidsolution in said bathing step electrolessly plates cobalt.
 6. A methodaccording to claim 1 wherein said solution in said bathing stepelectrolessly plates nickel.
 7. A method according to claim 1 whereinsaid dipping step lasts from five to fifteen seconds and said immersingstep lasts from fifteen to sixty seconds.
 8. A method of coating thesurfaces of spaced apart conductors which lie on one layer of polyimidewith a corrosion resistant metal without also coating the surface ofsaid polyimide between said conductors; said method including the stepsof:activating said surfaces of said conductors by immersing them into asolution of a catalyst for electrolessly plating said metal; immersing,subsequent to said activating step, all of said surfaces into anothersolution which includes a means for deactivating said polyimide surfacewhile keeping said conductor surfaces active; and bathing all of saidsurfaces, subsequent to said immersing step, in an electroless platingsolution for said metal.
 9. A method according to claim 8 wherein saidmeans for deactivating is a hydroxide.